This invention relates to optical modulators.
More particularly, the present invention relates to Mach-Zehnder modulators.
Electro-optic modulators and switches are important devices in integrated optical circuits such as fiber optic communication systems. A common optical switch is based on Mach Zehnder (hereinafter referred to as xe2x80x9cMZxe2x80x9d) modulators. These switches allow light to travel through the waveguide circuit and be reconfigured electronically at high speed and behave as the electrical to optical interfaces on the transmitter end of fiber optic links. MZ modulators are ideally built with III-V semiconductors, LiNbO3, or optical polymers. In particular, MZ modulators are important because they can be integrated with other optical devices, such as semiconductor lasers, optical amplifiers, or other electronic circuits. High-speed modulators are an integral component in most optical networks currently deployed worldwide. However, the problem with semiconductor MZ modulators in the prior art is that they have a high insertion loss and a low throughput and are also expensive to manufacture. These devices are expensive and difficult to manufacture because of the high performance and tight manufacturing tolerances required for most commercial applications.
Semiconductor Mach-Zehnder modulators work by shifting the optical absorption edge of the semiconductor under the influence of an external voltage. The shift in absorption with voltage also implies a change in refractive index with voltage. For example, this refractive index variation can for a typical modulator geometry can described by Equation 1:                               Δ          ⁢                      xe2x80x83                    ⁢          n                =                              1            2                    ⁢                      n            3                    ⁢                      r            eff                    ⁢                      xe2x80x83                    ⁢                      V                          d              xe2x80x2                                                          (        1        )            
where xcex94n is the change in index of refraction, n is the index of refraction of the material, reff is the effective electro-optic coefficient, V is the voltage applied across the electrodes, and d is the separation between the electrodes. Examples of materials that have large effective electro-optic coefficients are the III-V semiconductors, such as GaAs and InP (particularly when utilizing reduced dimensionality structures such as multiple quantum wells) and dielectric materials such as nonlinear polymers and LiNbO3. 
Semiconductor Mach Zehnder modulators typically consist of integrated optical waveguide circuits having a multiple quantum well (hereinafter referred to as xe2x80x9cMQWxe2x80x9d) layer structure. Incident light is separated into individual branches at the input coupler. A voltage of opposite polarities can be applied at the phase shift regions to each individual branch to change the index of refraction of the waveguide, and, consequently, the speed of the light traveling through each waveguide. As a result, the phase of the light signal is increased in one branch and decreased in the other. When the resulting out of phase light signals are recombined at the output coupler, the resulting light signal will have its intensity modulated.
MZ modulators described in the prior art typically use either Y branches or multimode interference (hereinafter referred to as xe2x80x9cMMIxe2x80x9d) couplers for both the input and output couplers. The problem with Y branch splitters is that small imperfections in the Y transition region can lead to very unequal 1xc3x972 splitting. Equal power splitting and combining are particularly important for achieving modulators with high contrast ratios. 1xc3x972 MMI couplers, however, always split the light signal equally and are ideally suited to be used as the input coupler. On the other hand, 2xc3x971 MMI couplers at the output are very sensitive to reflections and must be manufactured to tight tolerances.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a new and improved Mach Zehnder modulator.
It is another object of the present invention to provide a new and improved Mach Zehnder modulator which has a more equal power splitting ratio.
It is further object of the present invention to provide a new and improved Mach Zehnder modulator which has a low insertion loss.
It is still a further object of the invention is to provide a new and improved Mach Zehnder modulator which allows the active tuning of the device performance.
A further object of the invention is to provide a new and improved Mach Zehnder modulator which has a high throughput.
A further object of the invention is to provide a new and improved Mach-Zehnder modulator which uses an external photodiode to monitor the optical output and uses the MMI coupler electrodes to provide feedback and actively control the insertion loss and contrast ratio of the device.
To achieve the objects and advantages specified above and others, an index tuned optical modulator apparatus is disclosed. The optical modulator includes light paths having phase shifting regions and an index tuned multimode light splitter including light input and output terminals. The light output terminals are connected to supply light to the light paths. Finally, a light output device is connected to receive light from the light paths and combine the received light at a light output terminal.
The index tuned MMI coupler is a MMI coupler positioned between electrodes so that an electric field can be applied and the index of refraction of the MMI coupler material can be controllably varied. By varying the index of refraction of the MMI coupler material, the effective length and width of the MMI coupler device can be controlled. This is important because the performance of an MMI coupler is extremely sensitive to the geometry of the device. In the priorart, variations in the device geometry caused the need to fabricate many devices. These devices would be tested until one was found that had the desired performance. This process is expensive and inefficient because many devices were being fabricated and tested and then were never used in actual applications.